The invention relates to a communication system comprising a communication bus, a master device, which is connected to the communication bus and which communicates with the communication bus in a master mode, and several slave devices, which can be connected to the communication bus and which communicate with the communication bus in a slave mode, to a method for controlling and monitoring a communication between a master device and slave devices by means of a communication bus, wherein message frames are sent by the master device by means of the communication bus to the slave devices, and if applicable responses of the slave devices are sent back to the master device by means of the communication bus, as well as to a communication module comprising at least one input port for connecting the communication module to a master device by means of a communication bus as well as a plurality of output ports to which the slave devices can be connected.
A communication system of the type mentioned at the start is known from DE-A-10 2009 049 378. This document relates to a bus-capable connection unit for the connection and for the control of at least one operating device in a bus-oriented programmable electrical installation with a control unit, with at least one controlling unit for the control of the at least one operating device connected to the connection unit, with a control bus for the connection of the at least one control unit to the controlling unit, wherein the control unit comprises a bus connection device for the connection of the connection unit to a bus line of the electrical installation. In this system, the control bus is designed for the transmission of energy and data between the control unit and the at least one controlling unit, wherein the control unit comprises an energy supply unit for the energy supply of the control unit and of the at least one controlling unit, and the control bus is designed for the parallel connection of several controlling units.
A further communication system as well as an additional method for operating such a system is known from DE-B-198 08 230. The communication system comprises a communication bus, several electrical devices connected to the bus, which communicate with the bus in a slave mode, as well as a PC as main or master processor, which is connected to the bus and which works in a master mode. Furthermore, the system comprises a first monitor processor, which is connected to the bus, and communicates with the bus in a slave mode, when the master processor is in operation, wherein the first monitor processor communicates with the bus in a master mode, when the master processor is not in operation.
The communication system also has to communicate with electrical elements or slaves, which are connected to the communication bus, when the PC or master is not capable of transmission.
In the known communication system, the master device and the slave devices are connected directly by means of an RS-485 port to the serial communication bus.
A serial communication bus is used in the system in order to exchange data between the automation devices. Here, a plurality of protocols can be used, wherein mostly the master-slave principle in the half-duplex mode is used.
Before such a network is ready for operation, communication settings have to be set, such as the speed, parity bits, number of stop bits, for example, for each device depending on the values defined for the system. Furthermore, a unique address must be associated with each device before said device can be operated in the network. It is only then that a further configuration via the network is possible.
To implement the configuration, most devices require a corresponding PC application. Since the devices are not ready to be operated in the network prior to the implementation of a suitable communication configuration, they have to be connected once in a point-to-point connection directly to the PC.
When selecting the communication settings to be used in the communication system the communication properties of all the devices that are possibly connected to the network have to be taken into consideration. The maximum possible speed within the network results from the smallest common denominator and it is thus predetermined by the “weakest” device.
Other communication settings, such as parity bits or number of stop bits, for example, also have to be the same for each device. Without establishing a common base with regard to the speed and to the other communication parameters, the devices are not compatible and thus they cannot be operated in the same network.
In the operation of the communication system, the master device sends queries (requests) in the form of message frames to the slave devices and it receives answers (responses) which are sent back by the slave devices. Each message frame received by a slave device has to be evaluated in the respective slave device with regard to its address and checksum (CRC, Cyclic Redundancy Check), in order to detect whether the message frame is intended for the respective slave device or not. In order to process each received message frame, the slave device must perform corresponding calculations. During this time period, other system tasks have to be interrupted.
When the master device sends a request, the latter device waits for a predetermined time span (timeout), in which an answer (response) is expected to be sent back by the slave device. This time span (timeout) can be configured in the master device. The timeout value must be set depending on the slave device which takes the most time to respond. This is typically a multiple of the time required for a regular transaction, wherein, for example, in the case of a regular transaction of approximately 50 ms, a timeout value of 1000 ms is usually set.
Consequently, the absence of a slave device, for example, by disconnecting the slave device or by means of an interrupted line, has massive consequences on the entire network time behavior. This can also have a negative influence on intact slave devices, which expect a recurring request, in that, if the repeating query does not take place due to the waiting of the disturbed participant, they fall into a safe state of their main function, and thus no longer fulfill their function properly.
It results from the above that the slave devices first have to be configured in a point-to-point mode with a PC, before they can be integrated in a network and addressed via said network.
In large distributed systems, it is very time consuming to visit the slave devices in order to configure them. This is inefficient both in the setting up of the communication system and also in the replacement of a defective device.
A further disadvantage is that a common communication base must be supported by all the devices that are possibly connected to the communication. If no such a common denominator exists with regard to speed, parity bits, number of stop bits, etc., the devices cannot be operated in the same network. In addition, the network performance is limited by the “weakest” device.
Independently thereof, absent or defective devices have a massive effect on the network timing. Intact devices can also be influenced negatively.